Innervation maintains the metabolic properties of skeletal muscle. Evidence suggests that such trophic influences of motor nerves are mediated in part by muscle activity and in part by neurotrophic substances carried by axonal flow. While the mechanisms by which trophic influence regulates muscle properties is unknown, evidence suggests that this regulation may be mediated by the control of muscle protein turnover. This laboratory has purified a protein from chicken sciatic nerves which has trophic effects on cultured muscle cells. Addition of this protein which we have termed neurotrophic factor (NTF) to muscle cultures increases protein synthesis, enhances morphological development and maintains well-differentiated muscle fibers for prolonged periods in culture. Preliminary experiments suggest that the trophic effects of NTF may be mediated by the specific regulation of muscle protein synthesis. This study will test the hypothesis that maintenance of muscle properties is accomplished through control of protein turnover as mediated at least in part by neurotrophic influence. Using chick embryonic muscle culture as a model system, this study will investigate the respective effects of neurotrophic influence and muscle activity on the turnover of selected marker proteins. Antisera will be elicited against purified actin (contractile apparatus), cytochrome c (mitochondria) and lactate dehydrogenase (cytosol) in order to establish immunoprecipitation procedures for these marker proteins. The turnover of these proteins, as well as of the acetylcholine receptor (sarcolemma) will then be investigated. The role of the muscle proteases in regulating protein turnover will also be determined. It is hoped that this study will (1) provide evidence indicating the degree to which neurotrophic influence and muscle activity interact in controlling muscle protein turnover, (2) uncover the metabolic pathways or loci involved in this regulation and (3) elucidate the role of proteolytic enzymes in this process.